Sigma-delta modulators can be used in order to implement analog-to-digital converters (ADCs). Sigma-delta modulators can also be used in order to convert a digital signal from one signal format (e.g. an n-bit signal) into another signal format, in particular a 1-bit stream. Such converters, in particular analog-to-digital converters, using a sigma-delta modulator, subsequently also referred to as a sigma-delta converter, can use oversampling with noise shaping to attain high quantization resolutions. In such sigma-delta converters, a digital filter limits a spectrum of a generated output bitstream to an intended bandwidth.
Discrete-time sigma-delta modulators sampling an input signal at a certain sampling frequency would require a bandlimited input signal to be supplied in order to avoid violating the sampling theorem. Otherwise, what is known as aliasing can arise, in which interference signals at higher frequencies in the input signal are “aliased” into a designed bandwidth of the output signal, i.e. the interference signals then arise within the desired bandwidth of the output signal. In many practical applications, the input signal is not sufficiently bandlimited, however, and also cannot be bandlimited in this way in a simple manner. This can then just lead to the aforementioned aliasing, which can interfere with a measurement, e.g. in measurement applications, and can also lead to interference in other applications.
A conventional solution to this problem is to use an active antialiasing filter that limits the input bandwidth. Another conventional approach is to provide a sampling clock signal with a jitter. However, this method cannot reliably eliminate interference close to the sampling frequency.